The present invention relates to a device for producing plasma in a vacuum chamber with the help of electromagnetic alternating fields, in which a rod-shaped conductor that is inside a pipe and is made of an insulating material is guided into the vacuum chamber wall. More particularly, the inner diameter of the insulating pipe is larger than the diameter of the conductor, and the insulating pipe is held in the wall of the vacuum chamber at one end and its outer surface is sealed across from the vacuum chamber. The conductor is connected to a source for producing the electromagnetic alternating field.
With a known device for producing plasma (DE 195 03 205) it is possible to produce plasma for surface treatments and coating techniques within a limited operating range (process area, gas pressure, microwave output). The known device essentially consists of a cylindrical glass pipe installed in a vacuum chamber and a metallic-conducting pipe is located in it. Atmospheric pressure exists in the inner space of the glass pipe. Microwave output is initiated on both sides by two power supplies and two metallic coaxial transmission lines--which consist of an inner conductor and outer conductor--through the walls of the vacuum process chamber. The absent outer conductor of the coaxial transmission line inside the vacuum process chamber is replaced by a plasma discharge, which is ignited and maintained by the microwave output under sufficient igniting conditions (gas pressure). The microwave output can come out of both metallic coaxial transmission lines and the glass pipe in the vacuum process chamber. The plasma surrounds the cylindrically shaped glass pipe from the outside and forms, together with the inner conductor, a coaxial transmission line with a very high attenuation coefficient.
With a steady microwave output fed on both sides, the gas pressure of the vacuum process chamber can be adjusted in such a way that the plasma visibly bums evenly along the length of the device where the outside conductor of the coaxial transmission line is missing inside the vacuum process chamber.
When the gas pressure in the vacuum process chamber is raised with a preset microwave output, experience indicates that the uniformity of the plasma along the device is, however, lost. The plasma in about half the distance between the power supply points of the device becomes optically weaker and can be completely extinguished beyond a certain pressure. The plasma line "tears apart" and the two resulting plasma portions withdraw in the direction of the power supplies when the pressure is raised further. Especially with long devices (e.g., 1 m or more) this effect leads to irregular plasmas and, as a result, to irregular vacuum processes. The plasma portions display high luminosity at the ends of the power supply near the walls and are weaker towards the middle.
The attenuation of the microwaves does not depend in any way on the position along the coaxial transmission line. Because the dielectric filler of the coaxial transmission line consists of air and the relative dielectric constant of the dielectric of the coaxial transmission line is constant over the length of the device, the magnitude of the attenuation depends only on the penetration depth of the microwave in the conducting surfaces. When the attenuation per unit of length is constant, this means that the net microwave output per unit of length emitted on the plasma is falling along the device toward the middle. Because the outside conductor consists of plasma, the conductivity cannot be exactly determined. Of course it depends on the plasma density and this in turn is, to a limited extent, a function of the microwave power density in the discharge area. It is presumably several magnitudes higher than in the case of metallic surfaces (-50 .mu.m) and not constant over the length of the device.
Furthermore, it has been shown that the known device is rarely capable of maintaining a plasma discharge at pressures lower than 8.times.10.sup.2 mbar. To raise the flexibility of the device it would be desirable to guarantee the operating conditions for a plasma discharge without the support of the magnetic field even at lower pressures.
An object of the present invention therefore is to improve the known device and to enable coating larger, flat, three-dimensional substrates.
It is a further object of the present invention to accomplish this with only one opening in the wall of the vacuum chamber in each case.